The present invention relates generally to compressors and more particularly to protective arrangements for such compressors, especially of the type which disable the compressor drive motor when the temperature of the discharge gas from the compressor exceeds some prescribed value.
Such thermal overload protection systems are known generally and typically comprise a temperature sensitive element to be influenced by the temperature of the region to be protected with the temperature sensitive element being operative to, for example, control an electric circuit or the like which will initiate compensatory action. Such compensatory action in the case of a compressor may consist of turning off the compressor drive motor.
One well-known and commercially available thermal overload protection system comprises a thermal sensing element in contact with the compressor drive motor coils for detecting an overload condition such as a stalled or locked rotor by responding to the overheated motor coil manifestation of the problem and by way of a solid state motor control circuit interrupting the supply of current to the motor.
Another known arrangement for thermal overload protection in a compressor is disclosed in U.S. Pat. No. 3,278,111 issued to Sidney A. Parker on Oct. 11, 1966. The Parker patent locates a thermostatic switch of, for example, the bimetallic element type in the discharge gas manifold of the compressor which manifold forms the connection between the several compressor cylinder outlets and may function as a gas muffling chamber. The Parker arrangement has the thermostatic switch contacts closed so long as the temperature of this discharge gas is below some prescribed value and has those thermostat contacts in series with the compressor drive motor or may employ the state of those contacts for controlling a control relay which in turn supplies or interrupts the current flow to the motor depending upon the relay state. A variation on this arrangement is illustrated in the more recent Parker U.S. Pat. No. 3,877,837.
Rather than locating a temperature sensor in the discharge gas manifold directly adjacent to the compressor, the applicant has in his copending application Ser. No. 670,646, filed Mar. 26, 1976, now U.S. Pat. No. 4,059,366, placed a temperature sensing device in a well, closely adjacent to and downstream from the compressor cylinder discharge valve, thereby obtaining a very accurate indication of the discharge gas temperature emanating from the particular cylinder being monitored. With the arrangement dislosed in the applicant's prior application, it is possible, but not probable, that the gas emanating from an unmonitored cylinder exceeds the prescribed temperature limits, while the gas emanating from the cylinder being monitored does not, and accordingly to be absolutely sure of no excessive discharge gas temperatures, each cylinder could be provided with a temperature sensor. Also with the applicant's prior arrangement the electrical circuitry must pass from the region of the compressor heads to the region of the drive motor, which according to present day fabricating techniques requires that this circuitry pass relatively near an annular region where two halves of the hermetic enclosure or casing are joined, typically by welding. Such welding dramatically raises the temperature in the area of the welded joint, and the electrical connections with the applicant's prior device needed a metallic shielding conduit to prevent heat deterioration of the insulation on the conductors. The positioning of a temperature sensor close to the motor and entirely within one half of the casing will remove the conductors from the weld region, thereby obviating the need for heat protection for those conductors during the assembly welding process.